Eutrophic lichens respond to multiple forms of N: implications for critical levels and critical loads research.

Epiphytic lichen communities are highly sensitive to excess nitrogen (N), which causes the replacement of native floras by N-tolerant, "weedy" eutrophic species. This shift is commonly used as the indicator of ecosystem "harm" in studies developing empirical critical levels (CLE) for ammonia (NH3) and critical loads (CLO) for N. To be most effective, empirical CLE and/or CLO must firmly link lichen response to causal pollutant(s), which is difficult to accomplish in field studies in part because the high cost of N measurements limits their use. For this case study we synthesized an unprecedented array of atmospheric N measurements across 22 long-term monitoring sites in the Los Angeles Basin, California, USA: gas concentrations of NH3, nitric acid (HNO3), nitrogen dioxide, and ozone (n = 10 sites); N deposition in throughfall (n = 8 sites); modeled estimates of eight different forms of N (n = 22 sites); and nitrate deposition accumulated on oak twigs (n = 22 sites). We sampled lichens on black oak (Quercus kelloggii Newb.), and scored plots using two indices of eutroph (N tolerant species) abundance to characterize the community-level response to N. Our results contradict two common assertions about the lichen-N response: (1) that eutrophs respond specifically to NH3 and (2) that the response necessarily depends upon the increased pH of lichen substrates. Eutroph abundance related significantly but weakly to NH3 (r2 = 0.48). Total N deposition as measured in canopy throughfall was by far the best predictor of eutroph abundance (r2 = 0.94), indicating that eutrophs respond to multiple forms of N. Most N variables had significant correlations to eutroph abundance (r2 = 0.36-0.62) as well as to each other (r2 = 0.61-0.98), demonstrating the risk of mistaken causality in CLE/CLO field studies that lack sufficient calibration data. Our data furthermore suggest that eutroph abundance is primarily driven by N inputs, not substrate pH, at least at the high-pH values found in the basin (4.8-6.1). Eutroph abundance correlated negatively with trunk bark pH (r2 = 0.43), exactly the opposite of virtually all previous studies of eutroph behavior. This correlation probably results because HNO3 dominates N deposition in our study region.

Patterns of diversity for fungal assemblages of biological soil crusts from the southwestern United States.

Molecular methodologies were used to investigate fungal assemblages of biological soil crusts (BSCs) from arid lands in the southwestern United States. Fungal diversity of BSCs was assessed in a broad survey that included the Chihuahuan and Sonoran deserts as well as the Colorado Plateau. At selected sites samples were collected along kilometer-scale transects, and fungal community diversity and composition were assessed based on community rRNA gene fingerprinting using PCR-denaturing gradient gel electrophoresis (DGGE). Individual phylotypes were characterized through band sequencing. The results indicate that a considerable diversity of fungi is present within crusted soils, with higher diversity being recovered from more successionally mature BSCs. The overwhelming majority of crust fungi belong to the Ascomycota, with the Pleosporales being widespread and frequently dominant. Beta diversity patterns of phylotypes putatively representing dominant members of BSC fungal communities suggest that these assemblages are specific to their respective geographic regions of origin.

Life in extreme environments: survival strategy of the endolithic desert lichen Verrucaria rubrocincta.

Verrucaria rubrocincta Breuss is an endolithic lichen that inhabits caliche plates exposed on the surface of the Sonoran Desert. Caliche surface temperatures are regularly in excess of 60 degrees C during the summer and approach 0 degrees C in the winter. Incident light intensities are high, with photosynthetically active radiation levels typically to 2,600 micromol/m(2) s(-1) during the summer. A cross-section of rock inhabited by V. rubrocincta shows an anatomical zonation comprising an upper micrite layer, a photobiont layer containing clusters of algal cells, and a pseudomedulla embedded in the caliche. Hyphae of the pseudomedulla become less numerous with depth below the rock surface. Stable carbon and oxygen isotopic data for the caliche and micrite fall into two sloping, well-separated arrays on a delta(13)C-delta(18)O plot. The delta(13)C(PDB) of the micrite ranges from 2.1 to 8.1 and delta(18)O(SMOW) from 25.4 to 28.9, whereas delta(13)C(PDB) of the caliche ranges from -4.7 to 0.7 and delta(18)O(SMOW) from 23.7 to 29.2. The isotopic data of the micrite can be explained by preferential fixing of (12)C into the alga, leaving local (13)C enrichment and evaporative enrichment of (18)O in the water. The (14)C dates of the micrite range from recent to 884 years b.p., indicating that "dead" carbon from the caliche is not a significant source for the lichen-precipitated micrite. The endolithic growth is an adaptation to the environmental extremes of exposed rock surfaces in the hot desert. The micrite layer is highly reflective and reduces light intensity to the algae below and acts as an efficient sunscreen that blocks harmful UV radiation. The micrite also acts as a cap to the lichen and helps trap moisture. The lichen survives by the combined effects of biodeterioration and biomineralization. Biodeterioration of the caliche concomitant with biomineralization of a protective surface coating of micrite results in the distinctive anatomy of V. rubrocincta.

The relation of H2 S release to SO2 , fumigation of lichens.

Hydrogen sulphide emission in lichens as a response to low concentration SO2 , fumigation was investigated. In an open flow-through system several lichen species were fumigated with 36 ppb SO2 , Two species were also fumigated with higher concentrations (72, 119, 122 and 198 ppb SO2 ,). Hydrogen sulphide emission was monitored concurrently by cryogenic trapping and analysis with gas chromatography. All tested species increased H, S emission significantly in response to fumigation with 36 ppb SO2 . Parmelina tiliacea (L.) Hale and Cladina rangiferina (L.) Wigg. released significantly more H2 S (0.098±0.015 and 0.073±0.013 pmol H2 S g-1 d. wt s-1 , respectively) than Parmelina quercina (Ach.) Hale, Ramalina menziesii Tayl. and Parmelia sulcata Tayl. (0.028 ± 0.01, 0.025±0.014 and 0.023±0.013 pmol H2 S g-1 d. wt s-1 , respectively). Release of H2 S in Hypogymnia physodes was enhanced by increasing SO2 concentrations up to 72 ppb SO2 . No significant difference in H2 S emission in the dark vs. in the light was found. Generally, no correlation was found between photosynthetic activity and H2 S emission for the tested species. Uptake of SO2 was similar for all species, at 24.7 ± 5.6 pmol SO2 g-1 d. wt s-1 in 36 ppb SO2 and increasing at greater SO2 concentrations. Therefore, H2 S-S release represents only 0.11-0.74% of SO2 -S uptake.

Photosynthetic patterns of Cetraria cucullata (Bell.) Ach. at Anaktuvuk Pass, Alaska.

The daily photosynthetic patterns of Cetraria cucullata were followed over the 1976 summer period at Anaktuvuk Pass, Alaska. With the exception of rainy peroids, the lichen exhibited a strong diurnal pattern with peak photosynthetic activity occurring between 0300 and 0700 h. This correllated with periods of maximal lichen water retention and the presence of direct solar radiation. When the lichen was moist, a strong gradient in photosynthetic activity was observed with no activity in the lichen bases and maximal activity in the lichen tips.